Military Assault Command Operations (commonly abbreviated as MACO) was a United Earth organization that existed as a part of the United Earth Military during the 2150s, prior to the founding of the Federation. The MACOs (pronounced "MAY-ko") were not associated with Starfleet. As of 2153, MACO tactics and technology were two to three years more advanced than those of Starfleet, but their combat expertise was initially limited to simulations conducted on Earth. (ENT: "Harbinger"; Star Trek Beyond)

Following the foundation of the United Federation of Planets, the organization was disbanded, with personnel being offered Federation Starfleet commissions on a discretionary basis. The Federation Starfleet, just as its United Earth predecessor, continued to distance itself from the notion of being a military organization, causing deep conflict for at least one former MACO who donned a Starfleet uniform. (Star Trek Beyond)

The patch was later used during the production of Star Trek Beyond, modified to feature the name of the USS Franklin rather than Enterprise. The MACO patch was stenciled on the sides of PX70 motorcycles used in the film [1] and was printed onto T-shirts given to the cast and crew. [2]

There have been no references to any separate Earth military after the final MACO appearance in 2161. There has only been one explicit mention of a Human holding an army rank after this point in history, and the officer in question – ColonelWest – was shown to be a member of Starfleet in 2293 wearing a vice admiral's rank (which would make him a lieutenant general in the army).

Star Trek Beyond co-writer Doug Jung once noted about the disbanding of MACO, "I thought it'd be kind of interesting [....] You can make some modern analogies in terms of like the CIA, which is – you're asking these people, you're training them their whole lives to see the world one way, and then you say, 'Oh, by the way, you're now obsolete, but we'd like you to do [this other thing].' It's tough." [3]

The novelization of The Expanse states that Captain Archer was initially uncomfortable with the notion of military personnel serving aboard Enterprise but reconciled himself to the idea, given that it seemed as if the MACOs would be beneficial to the search for the Xindi weapon. The same novelization also suggests that MACO be pronounced "Mah-co" rather than "May-co", despite the latter being the commonly used canon pronunciation.

In Star Trek Online, the MACOs are an elite Starfleet force, using the standard Starfleet ranks, rather than the army/marine style. They have allied with the Klingon Honor Guard to form Task Force Omega, for the purpose of fighting the resurgent Borg Collective. Starfleet player characters can obtain specialized MACO armor and weaponry through the completion of Special Task Force missions against the Borg. It is widely considered to be some of the most powerful ground equipment available.